I agree with all the materials mentioned. Craft foam, as mentioned, would be very simple to use for this. But I would say plastic (I believe that sintra stuff is the same), and you could make the "seam" where the chain goes because you can use the section the chain is on to cover the seam. You can glue it together on one side, then on the other use a flat magnet to hold it together without problems. That way it all looka like one piece, but you can still take it off.

I would use craft foam and coat it with the hard finish kind of Mod Podge. If you want it to be really durable, though, you could layer Wonderflex or Worbla (if you're not on a budget) or paper mach (if you are on a budget) to make it sturdier.